1. Field of the Invention
The present invention relates to an optical amplifier mainly used in an optical communication system and suitable for amplifying a wavelength-division multiplexed optical signal having a band of 1.5 xcexcm.
2. Related Background Art
In optical fiber communication systems, rare earth doped optical fiber amplifiers (referred to merely as xe2x80x9coptical amplifiersxe2x80x9d hereinafter) have remarkably been developed. Particularly, a D-WDM system in which a wide amplifying band of the optical amplifier is utilized and a communication capacity is increased by using wavelength-division multiplexed optical signals obtained by multiplexing a plurality of optical signals in the amplifying band has mainly been progressed. However, although the optical amplifier has the wide amplifying band, an amplifying property thereof has wavelength dependency, input intensity dependency and temperature dependency.
Thus, when the wavelength-division multiplexed optical signals are amplified collectively, there arises a problem regarding difference in gain between the respective different optical signal wavelengths (referred to as xe2x80x9cchannelsxe2x80x9d hereinafter). That is to say, in the D-WDM system, when the optical amplifiers are connected in a multi-stage fashion, the gain differences between the channels are accumulated, thereby causing great output signal intensity difference ultimately. Since the transmission property of the entire optical transmitting system is limited by a channel having minimum output signal intensity, even when there are channels having greater output signal intensity, the transmission property of the entire optical transmitting system will be reduced.
To solve this problem, various techniques have been developed. As one of these techniques, there is means for controlling a temperature of the entire rare earth doped optical fibers to keep the temperature constant in order to eliminate the temperature dependency. However, the means for controlling the temperature of the rare earth doped optical fibers increases power consumption and makes the entire system bulky, and, increase in the used temperature range results in additional increase in power consumption.
Further, there is gain constant control means for keeping the gain constant by adjusting output intensity in accordance with input intensity after gain spectrum is flattened by inserting a correction filter into an optical amplifier portion in order to eliminate the wavelength dependency from the amplifying property. FIG. 44 shows an example of an optical amplifier utilizing such means. In the optical amplifier shown in FIG. 44, optical fiber amplifiers are connected in a two-stage fashion. The optical amplifier comprises an input optical connector 1a, an output optical connector 1b, optical couplers or beam splitters 2a, 2b, 2c, 2d, optical monitors PD 3a, 3b, 3c, 3d, optical isolators 4a, 4b, 4c, 4d, pumping light/optical signal wavelength-division multiplexers 5a, 5b, 5c, pumping light sources 6a, 6b, 6c, rare earth doped optical fibers (optical fiber amplifiers) 7a, 7b, an optical variable attenuator 8, and optical signal gain constant pumping light source control circuits 9a, 9b. In this optical amplifier, a part of input optical signal outputted from the input optical connector 1a is picked up by the beam splitter 2a and light intensity thereof is measured by the optical monitor PD 3a. The optical signal passes through the optical isolator 4a and is incident on the optical fiber amplifier 7a which is now maintained in a pumping condition by the pumping light source 6a. In this optical fiber amplifier, the optical signal is subjected to optical amplification by stimulated emission. The optical-amplified optical signal passes through the optical isolator 4b, and a part of the light is picked up by the beam splitter 2b and light intensity thereof is measured by the optical monitor PD 3b. The pumping light source 6a is adjusted by the optical signal gain constant pumping light source control circuit (AGC) 9a so that a ratio between the input optical signal of the optical monitor PD 3a and the output optical signal of the optical monitor PD 3b becomes a constant value. The optical signal passed through the first stage passes through the optical variable attenuator 8 and is incident on the second stage. The second stage is operated in the similar manner to the first stage, so that the signals of the optical monitors PD 3c, 3d are compared by the optical signal gain constant pumping light source control circuit (AGC) 9b, and the pumping light sources 6b, 6c are controlled so that a ratio therebetween becomes a constant value. As a result, even if the light intensity of the input signal is changed, gain spectrums of the optical fiber amplifiers in the first and second stages are kept constant.
However, in the optical amplifier utilizing the gain constant control means as shown in FIG. 44, since the intensity of the pumping light is varied with the light intensity of the input signal, in a small input optical signal area within the operation input optical signal intensity range, the intensity of the pumping light becomes small, thereby deteriorating noise figure. Further, since the intensity of the pumping light is greatly changed, the first stage of the gain constant control requires forward pumping or bi-directional pumping.
In consideration of the above, an object of the present invention is to provide an optical amplifier of multi-stage type having a plurality of rare earth doped optical fibers and in which temperature dependency of gain spectrum can be compensated so as to be operated with constant gain spectrum regardless of used temperature. Another object of the present invention is to provide an optical amplifier which can be operated with constant gain spectrum regardless of intensity of input optical signal, insertion loss of parts between stages and light intensity of output optical signal. A further object of the present invention is to provide an optical amplifier in which noise figure is improved in a small optical signal area within an operation input optical signal range, and a gain configuration is kept constant regardless of intensity of input optical signal, and output variable control can be performed.
According to a first aspect of the present invention, there is provided an optical amplifier having a plurality of rare earth doped optical fibers in a multi-stage and comprising one or more optical variable attenuator means, and an attenuation amount control means for changing an optical attenuation amount of the optical variable attenuator means on the basis of temperature of the rare earth doped optical fibers or an environmental temperature.
According to a second aspect of the present invention, there is provided an optical amplifier having a plurality of rare earth doped optical fibers in a multi-stage and comprising, a replaceable optical part between the rare earth doped optical fibers, one or more optical variable attenuator means, and an attenuation amount control means for changing an optical attenuation amount of the optical variable attenuator means on the basis of temperature of the rare earth doped optical fibers or an environmental temperature.
According to a third aspect of the present invention, in the optical amplifier according to the first or second aspect, the attenuation amount control means has an optical attenuation amount table associated with the temperature, and the optical attenuation amount of the optical variable attenuator means is changed on the basis of the optical attenuation amount table.
According to a fourth aspect of the present invention, in the optical amplifier according to the third aspect, when an equation obtained by applying the regression line based on the method of least squares to the optical attenuation amount table is represented by xe2x80x9coptical attenuation amount=coefficient A [dB/xc2x0 C.]xc3x97temperature [xc2x0 C.]+any coefficientxe2x80x9d, the coefficient A is selected to be within a range from xe2x88x920.16 [dB/xc2x0 C.] to +0.26 [dB/xc2x0 C.].
According to a fifth aspect of the present invention, in the optical amplifier according to the third aspect, a wavelength band of optical signal inputted to the optical amplifier is 1580 to 1590 nm, and, when an equation obtained by applying the regression line based on the method of least squares to the optical attenuation amount table is represented by xe2x80x9coptical attenuation amount=coefficient A [dB/xc2x0 C. ]xc3x97temperature [xc2x0 C.]+any coefficientxe2x80x9d, the coefficient A is selected to be within a range from xe2x88x920.16 [dB/xc2x0 C.] to xe2x88x920.04 [dB/xc2x0 C. ].
According to a sixth aspect of the present invention, in the optical amplifier according to any one of first to fifth aspects, the attenuation amount control means changes the optical attenuation amount of the optical variable attenuator means by using one or plural or all of intensity of input optical signal to the optical amplifier, an insertion loss amount of the replaceable optical part and intensity of output light from the optical amplifier, as well as the temperature.
According to a seventh aspect of the present invention, there is provided an optical amplifier having a plurality of rare earth doped optical fibers in a multi-stage and comprising one or more externally controllable optical variable attenuator means, and an optical attenuation amount of the optical variable attenuator means is varied with intensity of input optical signal to the optical amplifier and intensity of output light from the optical amplifier.
According to an eighth aspect of the present invention, in the optical amplifier according to the seventh aspect, an optical fiber amplifier in a first stage is subjected to pumping light intensity constant control or pumping current constant control, and optical fiber amplifiers in stages other than the first and last stages are subjected to any control other than gain constant control, and gain spectrum of the entire optical amplifier is made constant by controlling an optical fiber amplifier in the last stage and the optical variable attenuator means between the optical fiber amplifier stages.
According to a ninth aspect of the present invention, in the optical amplifier according to the seventh or eighth aspect, it comprises an optical attenuation amount table associated with the intensity of input optical signal to the optical amplifier and the intensity of output optical signal from the optical amplifier, and the optical attenuation amount is changed in accordance with the table.
According to a tenth aspect of the present invention, in the optical amplifier according to the ninth aspect, a relationship between the intensity of input optical signal to the optical amplifier, intensity of output optical signal from the optical amplifier and optical attenuation amount of the optical variable attenuator means is represented by xe2x80x9coptical attenuation amount=coefficient A xc3x97(intensity of output optical signal from the optical amplifierxe2x88x92intensity of input optical signal to the optical amplifier) [dB]+any coefficientxe2x80x9d, and the coefficient A has a value within a range from xe2x88x920.8 [dB/dB] to xe2x88x921.1 [dB/dB].
In the optical amplifier according to the present invention, the rare earth doped optical fibers in the last stage may be subjected to output constant control.
In the optical amplifier according to the present invention, the rare earth doped optical fibers in the first stage may be subjected to pumping light output constant control.
In the optical amplifier according to the present invention, the attenuation amount control means may have an optical attenuation amount table associated with the intensity of input optical signal to the optical amplifier, insertion loss amount of the replaceable optical part and intensity of output optical signal from the optical amplifier, and the optical attenuation amount of the optical variable attenuator means may be changed on the basis of information derived from the table and the temperature.
In the optical amplifier according to the present invention, when an equation obtained by applying the regression line based on the method of least squares to the optical attenuation amount table associated with the intensity of input optical signal to the optical amplifier, insertion loss amount of the replaceable optical part and intensity of output optical signal from the optical amplifier is represented by xe2x80x9coptical attenuation amount=coefficient B [dB/dB]xc3x97(intensity of output optical signal from the optical amplifierxe2x88x92intensity of input optical signal to the optical amplifier+insertion loss amount of the replaceable optical part)[dB]+any coefficientxe2x80x9d, the coefficient B may be selected to be within a range from xe2x88x920.8 [dB/dB] to xe2x88x921.2 [dB/dB].